We are using optical detection of magnetic resonance, odmr, to investigate the binding of methylmercury, a spin orbit probe to specific target sites in living cells. At present, simple target molecules are under investigation, including mononucleotides, mononucleosides, synthetic DNA duplexes, phage DNA, aromatic amino acids, simple peptides, and enzymes. Methylmercury binding to or in the proximity of an aromatic chromophore leads to an external heavy atom effect, usually resulting in an enhanced triplet yield and phosphorescence quantum yield. These allow enhanced sensitivity for odmr detection which therefore is selective for the perturbed chromophore. Shifts in the zero field splittings are sensitive to, and thus help to identify the mode of binding to the target molecule.